The present invention relates to an infrared detector and a drive method therefor, and in particular, to an infrared detector with a wide range of applications such as to a crime prevention, watching, guidance, medicine and industrial instrumentation, and to a drive method therefor. More specifically, the invention relates to a thermal infrared detector having a thermoelectrical conversion element for converting thermal energy into an electrical signal, and to a drive method therefor.
Description of the Related Art
Recent years have observed an increased demand for an infrared detector adaptive to measure a temperature of an object in a non-contact manner, with an increased need for development of an inexpensive high-performance thermal infrared detector.
The state of art will be described with reference to FIGS. 1A and 1B.
FIGS. 1A and 1B show a sectional view and a plan view of a conventional thermal infrared detector, respectively.
As shown in the figures, the conventional thermal infrared detector includes a semiconductor substrate 29, a scan circuit 30 formed thereon, and a cavity 31 formed above the circuit 30, and further has formed over the cavity 31 a diaphragm or beam with a thermoelectrical conversion element 32 and an infrared absorption layer 34 thereabove.
The thermoelectrical conversion element 32 comprises a titanium bolometer made of the titanium, a material with a variable resistance depending on a temperature.
The diaphragm is constituted with three layers: the titanium bolometer 32, a silicon oxide film 33 and a titanium nitride film as an infrared absorption layer 34. As the silicon oxide film 33 set to a thickness of .lambda./(4n) (n is a refractive index of the silicon oxide film, and .lambda. is a wavelength of an incident infrared ray), correspondent infrared rays reflected by the titanium bolometer 32 are absorbed into the titanium nitride 34.
This is due to an electromagnetic effect in which an electromagnetic wave of a wavelength .lambda. constitutes a standing wave resonating in the gap of .lambda./(4n). The titanium bolometer 32 concurrently serves as a thermoelectrical conversion element and an infrared reflection film.
The diaphragm is formed over the cavity, with long legs to avoid a dissipation of thermal energy, so the absorbed infrared rays cause a temperature rise over an entirety of the diaphragm. This leads to a resistance variation of the titanium bolometer 32 in the diaphragm, which variation is externally output through a scan circuit as discussed, e.g. in "Infrared Absorption of Thin Metal Films" by C. Hilsum, JOURNAL OF THE OPTICAL SOCIETY OF AMERICA, VOL. 44, NO. 3, 1954, and Japanese Patent Application No. 6-189144.
The conventional infrared detector described uses a combination of a titanium nitride film, a silicon oxide film and a titanium bolometer (reflection film) as an infrared absorption layer.
In another conventional case, a vacuum is used instead of the silicon oxide film to form a gap of .lambda./(4n), as disclosed e.g. in Japanese Patent Laid-Open Publication No. 2-196929.
In this case, an aluminum reflection film is provided under a cavity, and a titanium nitride as an infrared absorption layer is provided inside a diaphragm on the cavity. In the case of vacuum, a refractive index n thereof is approximately 1, so the cavity gap is set to .lambda./4.
Further, in this conventional case, a chopper is provided at a front side of an infrared detector, to periodically cut off incident infrared rays.
In general, the chopper is employed to temporarily cut off incident infrared rays for a calibration of an offset (drift) of an infrared detector, as the offset is variable for various reasons. For example, in a thermal infrared detector with a bolometer, a temperature variation of the detector itself may turn into a signal as it is, giving an adverse effect. When objects having a temperature difference of 1.degree. C. are observed, a temperature variation of about 0.002.degree. C. may well be caused at a diaphragm by incident infrared rays. Therefore, even if the temperature variation of the detector is very small, the resultant effect may be large.
A signal output from the infrared detector is typically subjected to several amplification stages and correction stages such as for removing a fixed pattern. There may also be large temperature drifts in circuits associated with such stages, giving eventual influences on the signal. Therefore, such influences are removed by a correction using as a reference a signal level obtained when incident infrared rays are cut off.
In the conventional infrared detectors, a wavelength for an optimum absorption depends on a gap distance. However, as an infrared absorption layer is fixed, a detection sensitivity may be lowered with a deviating variation in wavelength of incident infrared rays.
For example, when observing an object having a temperature of approximately 300.degree. K., which is close to a room temperature, a wavelength with maximum power appears in a vicinity of 10 .mu.m, while it shifts in a vicinity of 4 .mu.m when a high temperature object of approximately 1000.degree. K. is observed.
In this respect, a gap distance set to e.g. 10 .mu.m/(4n) is optimal for absorbing a wavelength of 10 .mu.m. However, for wavelengths deviated from 10 .mu.m, an optimum absorption is unattainable.
Further, as a chopper periodically cutting off incident infrared rays is typically driven by a driver such as a motor, there are additional problems in size reduction and power consumption. However, without a chopper, an infrared detector is attended with another problem that a large temperature drift occurs.
The present invention has been achieved with such points in mind.